High tensile strength steel sheets are often used as materials for component parts of automobile bodies for fuel efficiency enhancement for prevention of global warming and enhancement of safety in car collision. From among the automobile body components, strength parts such as side sills, side members and bumper reinforcements are required to have a high crashing load with respect to a three-point bending load. These strength parts generally have an elongated and a complex shape to prevent interference with other parts and secure spaces for disposing other parts.
The formability of a steel sheet decreases as the strength of the steel sheet increases. For example, when a high tensile strength steel sheet is used for an elongated press-formed product (for example, a side sill inner panel) having a hat-shaped cross section, which is manufactured by press forming, springback tends to occur because of the strength. The occurrence of springback in a press-formed product results in a failure and a reduction of yield in the subsequent manufacturing steps (for example, the welding step). Thus, there is a high demand for the suppression of springback of press-formed products using a high tensile strength steel sheet as a starting material.
FIG. 17 is an explanatory view schematically showing an example of a press forming apparatus 1 performing conventional bending. FIG. 18 is an explanatory view schematically showing springback occurring in shoulder portions 7c of a press-formed product 7.
As shown in FIGS. 17 and 18, the press forming apparatus 1 typically includes a punch 2, a die 3, and an upper pad 5 installed in such a manner that the upper pad 5 is embedded in the die 3 through a pressing member 4 so as to freely advance from, and retract into, the die 3. The pad may be installed as a lower pad in the punch 2, rather than installing the pad in the die 3 as the upper pad 5 like in the press forming apparatus 1 shown in FIG. 17.
A steel sheet 6 is bent and formed by the press forming apparatus 1 into a press-formed product 7 having a hat-shaped cross section. The bending is performed by making the upper pad 5 abut against a part of the press-formed product 7 that is to be formed into a bottom portion 7a before the forming, and then lowering the die 3 while maintaining the abutting as shown in FIG. 17. The bottom portion 7a of the press-formed product 7 is formed along a punch upper surface 2a of the punch 2. Wall portions 7b of the press-formed product 7 are formed along punch side surfaces 2b of the punch 2. Furthermore, curved-shouldered portions 7c of the press-formed product 7 that are contiguous to the bottom portion 7a and the respective wall portions 7b are formed along punch shoulder portions 2c of the punch 2.
Upon the steel sheet 6 being bent, tensile stress is generated on a front surface of the steel sheet 6 and compressive stress is generated on a back surface of the steel sheet 6. Accordingly, bending the shoulder portions 7c of the press-formed product 7 along the punch shoulder portions 2c consequently generates tensile stress on a front surface and compressive stress on a back surface. The generated tensile stress and compressive stress are released when the press-formed product 7 is released from the press forming apparatus 1. Consequently, springback occurs in the shoulder portions 7c of the press-formed product 7, whereby the press-formed product 7 has a shape in which the wall portions 7b spread outward as indicated by the circular arc-shaped arrows in FIG. 18, which is different from that of the die.
Patent Documents 1 to 4 each disclose a method for suppressing an amount of an angle variation (amount of springback) of a press-formed product using a lower pad of a press forming apparatus.
Patent Documents 5 and 6 disclose a method for enhancing crash performance of an automobile by quenching a part of a strength part of an automobile body by high frequency induction heating to be hardened and reinforced. Also, Patent Document 7 discloses a method for enhancing bending deformation performance of a hat-shaped strength part by reinforcing shoulder portions of the strength part by overlaying.